The invention relates to apparatus for scraping the inside surface of a well casing, and is particularly useful for oil well casings.
An oil well casing, is subject to having its interior surface coated with residue and other substances. Such coating may significantly reduce the effective diameter of the casing, and interfere with proper working of the well, and may also interfere with access to the well through the casing. Therefore, the accumulated residue coating must occasionally be scraped off the interior surface of the well casing.
A variety of devices have been developed for performing that scraping function. A typical scraper scrapes the interior surface of the well casing as it is pushed down through the casing bore hole. After the scraper device has traveled the length of the casing and removed the buildup of residue from the casing walls, it is withdrawn by pulling it upward through the well casing.
To effectively scrape the casing walls, the blades of the scraper must be held firmly against the interior casing surface during the scraping operation. However, when the scraper is removed from the well, the blades should be permitted to withdraw radially in the event an obstruction is encountered, so the scraper does not become stuck in the casing.
One example of a casing scraper previously used is described in U.S. Pat. No. 4,291,764, issued Sept. 29, 1981 to Davy G. Pampell. This patent describes a scraper in which a central mandrel has scraping blades spring biased radially outward to contact the casing walls. The scraping blades and biasing springs are mounted on sloped ramp surfaces on the mandrel, so axial movement of the blades changes their radial position. An adjustable upper collar limits upward axial movement of the scraping elements. A spring biased lower collar controls downward axial movement of the blades. The continued contact of the scraping blades with the wall of the casing during the scraping action is dependent upon the biasing springs holding the blades radially outward. If a particularly heavy residue coating is present on the casing, the biasing springs may yield to the coating, and the blades will not completely remove the coating from the casing.
The problem of maintaining positive contact with the casing wall is solved somewhat by a structure shown in U.S. Pat. No. 3,251,418, issued May 17, 1966 to E. L. Condra. The Condra device ensures positive contact between the scraper element and the wall of the casing by mounting the scraping elements on a cam surface. As the scraper is moved downward through the bore hole during the scraping operation, the scraping elements are wedged against the cam surface to prevent them from moving radially inward. The outward radial position of the scraping element is controlled by a thin adjustable upper guide that limits the vertical movement of the scraping element on the cam surface. A lower collar and biasing spring allows controlled downward and radially inward movement of the scraping elements when an obstruction is encountered as the device is withdrawn upwardly through the bore hole. The Condra device, however, relies on a fairly complicated and intricate adjustment mechanism and appears to have limited protection against circumferential movement of the blades during scraping. Additionally, the device has a limited range of adjustment of the radial position of the scraper elements.